Fix an fauly setState when container could be unmounted in future

Author: darekg11

src/ImageHeaderScrollView.js

@@ -197,7 +197,11 @@ class ImageHeaderScrollView extends Component<Props, State> {
     if (!this.container) {
       return;
     }
-    this.container.measureInWindow((x, y) => this.setState(() => ({ pageY: y })));
+    this.container.measureInWindow((x, y) => {
+			if (this.container) {
+				this.setState(() => ({ pageY: y }))
+			}
+		});
   };
 
   onScroll = (e: *) => {